The terminal airways of the lung are lined with an acellular layer of material which appears vital in the maintenance of normal pulmonary function. The composition and origins of the acellular lining layer are being investigated. Current attention has been directed towards: (1) the biosynthesis and secretion of pulmonary surfactant, and (2) the acellular lining layer as an actively metabolizing medium. The objectives of this investigation are as follows: (1) to determine the enzymatic and protein composition of the lung lining, (2) to determine the origin of these activities, and (3) to investigate compositional variations and biosynthesis of the lining under the influence of drugs, disease, and airborne pollutants.